Control system



Patented Sept. 27 1938 'unrrao STATES PATENT OFFICE CONTROL SYSTEM Allan J. Wade, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application December 15, 1937, Serial No. 179,887 11 Claims. (Cl. 172-179) This invention reiates to control systems, more particularly to systems for controlling the stopping of reversiieie alternating current motors, and it has for an object the provision of a simple, reliable and improved system of this character.

Heretolore, control systems for reversible alternating current motors have been proposed wherein provision was made for stopping the motor and its load by plugging the motor. A H) motor is plugged when its primary winding'is connected to the power source for rotation in a direction opposite to that in which its rotor is actuallyrotating. This occurs, for example, when a motor is rotating and driving its load in the 15 forward direction, and the connections between the motor and the source are suddenly reversed.

When a motor is plugged, a large reverse torque is developed which quickly brakes the motor and its load to rest. After stopping the load, the 90 motor will start to. drive the load in the reverse direction unless disconnected from the source at the proper instant. In many industrial and other applications, such a reversal of the load is undesirable and may lead to damage to ex- 25 pensive machinery or other serious accidents.

For the purpose of overcoming this difficulty, various forms of control switches operated either frictionally or electromagnetically by the driving gears, have been arranged to disconnect the 30 motor upon initial reverse movement of the load.

Devices of this character require special drive shaft extensions which, in turn, require space which often is not available. Such devices are also subject to vibrations which adversely affect their operation and produce rapid deterioration. Furthermore, such devices are not suitable for vertical motors without a considerable amount of extra gearing and other complications. Electroresponsive devices connected in the 40 secondary circuit of an alternating current motor have also been utilized for this purpose, but the accuracy of these devices was adversely affected by variations in the line voltage and by the frequency effect of the variable frequency alter- 5 nating voltage supplied by the secondary wind ings to the actuating coils of such devices. In this connection, the secondary voltage and frequency vary in direct proportion and as a result the current supplied to the actuating coil of an 50 electroresponsive device remains substantially constant over a wide range of voltage and frequency', thereby rendering the electroresponslve device insensitive to variations in the speed of the motor. 55 Consequently, an important object of this in vention is the provision of a novel arrangement of control apparatus and circuits whereby an alternating current motor may be plugged, i. e. reverse power braking connections established for the motor, and a simple and reliable electro- 5 responsive device provided for interrupting the reverse power braking connections at substantially zero speed of the motor and'in which provision is made for eliminating inaccuracies resulting from variations in line voltage and from frequency effects.

In carrying the invention into effect in one form thereof, means are provided for connecting the motor to a source of alternating voltage for rotation in the forward direction, and means are provided for disconnecting the motor from the source and reconnecting it therewith for rotation in the reverse direction, thereby establishing reverse power braking connections for the motor.' In addition, means responsive to the voltage of the primary and secondary windings are provided for interrupting the reverse power braking connections at substantially zero speed of the motor. In order to eliminate the frequency effects from the voltages supplied to this voltage responsive means, suitable rectifying means are provided in the connections.

In illustrating the invention in one form thereof, it is shown as applied to a wound rotor induction motor suitable for driving a load which must be stopped quickly such, for example, as a large sugar centrifugal or the supercalender of a paper-finishing machine. It will be understood, however, that the invention has other applications and may be applied to other types of motors.

For a better and more complete understanding of the invention, reference should now be had to the following specification and to the accompanying drawing in which Fig. 1 is a simple, diagrammatical representation of an embodiment of the invention, and Fig. 2 is a simple, diagrammatical illustration of a modification of the system of Fig. 1.

Referring now to the drawing, a load, such for example as a sugar centrifugal (not shown) is driven by an alternating current motor l0, illustrated as a wound rotor induction motor. Motor I0 is supplied with power from a suitable source, represented by the three supply lines II to which the motor is connected for rotation in the forward direction by means of .an electromagnetic con-' tactor [2 under the control of a suitable master "switch ii, The master switch 13 is illustrated as comprising a pair-of stationary contacts Ii.

and a movable contact l3s for bridging the stationary contacts.

For the purpose of plugging the motor, i. e. establishing reverse power braking connections therefor, and electromagnetic contactor I4 is provided for connecting the primary winding of motor Hi to the source H for rotation in the reverse direction. For the plugging operation, 'both contactors i2 and M are under the control of a control relay l5 which, in turn, is under the control of a. suitable stop push button type switch 15.

A plugging resistor i1 is included in the secondary circuit oi the motor W for limiting the current inrush to the motor during the plugging operation. An electromagnetic contactor it is provided for short-circuiting this resistor when the motor is started from rest.

A resistor it known as an accelerating resistor is also including in the secondary circuit of the motor, and an electromegn centactor provided for short-siren he acceleratin sister it during the starting operation 0; motor.

For the purpose of controlling the reverse con tactor M to interrupt the reverse power l v liing connections at substantially zero speed motor, suitable elec rorespcnsive means provided, These means are iilustrated as a having a core member tie upon which two cctu ating coils Us and ale are wound. relay shown as provided with a pair of stationary torrtacts ii in and an armature member Zia carrying a cooperating movable contact member H; which is biased to the open position by a spring U In order to eliminate any inaccuracies in the operation of the electroresponsive 2! re" sulting from variations in the line voltage, one of the actuating coils is connected to the primary winding of the motor Hi and the other of the actu ating coils is connected to the secondary winding Obviously, this connection oi the actuating coil to the secondary winding oi the motor also ineludes connection to a special small secondary winding on motors such as squirrel-cage motors which do not have secondary working windings, The actuating coils 2h, and Me are so connected that their magnetomotive forces oppose each other. Since any variation in the line voltage is reflected in the secondary voltage of the motor, any inaccuracy or the relay as a result or" varia tion in the line voltage is balanced out the connections described. As the actuating cell We is connected through it open intcrlocks M. on the reverse contact i i to one phase of the secondary terminals he motor i3. Similarly the actuating coil ts relay 2! is connected by means of conducts 25 and 2E and the normally open contacts is or relay 26 to the primary winding terminals motor or to the source as shown.

In order to eliminate the effects oi varying frequency of the variable frequency voltage supplied to the actuating coil 2%, suitable full wave rectifying means 21 are included in the connections between the actuating coil and the secondary winding of the motor iii, and a similar rectifying means 28 is included in the connections between the actuating coil Ric and the source. Although any suitable rectifying means may be employed, the rectifying devices 2'! and are iiiustrateti as copper oxide type rectifiers connected. in Bridge circuit connections to provide full wave rectification. Suitable resistors 29 3B are included in the coil connections, and a suitable adjustable lugging contactor i8.

is readily traced from the lower supply line i l to resistor 3| is included in the connections 26 for .the purpose of initially adjusting the balance ofv the coils Us and 2h.

A lockout relay 32 is provided to prevent restarting the motor lfl until the master switch I3 has been operated to the off position after a plugging operation.

With the foregoing understanding of the elements and apparatus and their organization in the system, the operation of the system itself will readily be understood from the following detailed description.

To start the motor ill, the master switch I3 is actuated to its closed position in which a circuit is established for the actuating coil of the forward contactor Hi. This circuit is traced from the lower supply line H, through conductor 33, i

contacts of master switch I3, conductors 34 and Elli, lower contacts of lockout relay 32 and control relay i5, conductor 36, actuating coil of contactcr iii to the upperslde oi the supply source ii. In. response to energization, oontactor it closes its contacts to connect the primary winding or"; motor iii to the supply source H for rotation in the forward direction. Simultaneously, an energiz-- ing circuit is established for the actuating coils of This circuit is the same as previously traced for contactor it. as for as the conductor 35 thence through conductors ST and 38, actuating coil of plugging contactcr i8 and conductor 39 to the upper side of the supply source ii. Plugging contactor Iii closes its c0ntacts in response to energization and short-sin cults the plugging section ll of the secondary resistor. At the same time the actuating coil of a time delay relay 40 which is connected in parallel with the actuating coil of contactor I8 is energized, and after an interval of time deter-- mined by the setting of the time delay device 40s, the time delay relay 40 closes its contacts to establish an energizing circuit for the actuating coil of the accelerating cont-actor 20. This circuit the conductor 31 as before and thence through the contacts of time delay relay 40 and actuating coil of accelerating contactor 2D to the upper side of the supply source ii. In response to en ergization, accelerating contactor 20 closes its contacts to short-clrcuit the accelerating resist-0r it, as a result of which'the speed of the motor is increased to full operating speed.

If it is desired to stop the motor and. to bring it rapidly to rest, the push button 46 depressed to complete an energizing circuit for the actuating coil of the control relay iii. In response to energization, control relay l5 closes its upper and intermediate normally open contacts and opens its lower normally closed contacts thereby to interrupt the circuit for the actuating coil of forward contactor i2, and also to interrupt the energizing circuit for contactors I8 and 20, and the time delay relay 40 previously traced through these contacts. As a result forward contactor I2 opens its contacts and vdisconnects the motor In from the supply source I l. Simultaneously contactor i8 opens its contacts and inserts the plugging resistor I1 in the secondary circuit of motor iii, and accelerating contactor 20 opens its contacts and inserts the accelerating resistor IS in the secondary circuit of motor to.

In the closed position of the control relay IS, a circuit is established for the actuating coil of the reverse contactor I4 which is traced from the a lower side of the supply source H through the contacts of the master switch I3, conductors 34,

motor.

' switch 35, and H, intermediate contacts of control relay I5, conductor 42, normally closed interlocks on the forward contactor I2, actuating coil of reverse contactor I4 to the upper side of thesupply source I I. In response to energization, contactor I4 closes its main contacts to connect the motor III to the supply source II for rotation in the reverse direction while its armature is still rotating in the forward direction. As a result of the establishment of these reverse power braking connections, a large braking torque is produced which tends to bring the motor III rapidly to rest. The magnitude of the current inrush to the motor and the magnitude of this braking torque are limited to safe values by means of the plugging resistor l1 and the accelerating resistor I9, both of which are now included in the secondary circuit of the In the closed position of the reverse contactor l4, the interlock I4- is also closed to connect the actuating coil 2Ib of relay 2I to one phase of the secondary winding of the motor I0.

At the instant the motor is plugged, the secondary voltage and frequency of motor III become double the locked rotor values and since the frequency effect of the voltage supplied to coil 2Is is eliminated by means of the rectifier 21, the magnetomotive force of coil 2Ib is also doubled. As a result a flux is set up through the core and armature member 2'1: of the relay, which causes the armature member to move the movable contact member 2|: to bridge the stationary contact 2 Id and the movable contact member 2 I j to bridge the stationary contact members 2h. of the relay against the tension of the spring Zl When the stationary contacts 2| 4 are bridged by the movable contact member 2I1, a. circuit is completed in parallel with the contents of the push button I 6 which may now be released. The closing of contacts 2h; completes the connection of coil 2Ic to the primary winding of motor Ill.

As the speed of the motor III is reduced, the magnitude of the secondary voltage and the magnitude of the magnetomotive force of the coil 2h, are correspondingly reduced. At substantially zero speed of motor ID the magnetomotive force of coil 2Ibbecomes equal to and balanced by the magnetomotive force of coil 2|c thereby reducing the flux in the core and armature of the relay to zero and allowing the spring 2|; to open the contacts 2Ie and 2Ih. Since the push button switch I6 has been released, relay 2I in its open position interrupts the energizing circuit for the control relay I5. As the result of deenergization, control relay I5 closes its lower contact and opens its upper and intermediate contacts to interrupt the energizing circuit for the reverse contactor I4. As a result of its deenergization, contactor I4 opens its main contacts to interrupt the reverse power braking connections for the motor III, and also opens its interlock contact I 4! to interrupt the connections for the actuating coil 2h, of relay 2I. The circuit of coil 2 Ic is interrupted at contacts 2 lb- When control relay I5 was actuated to the closed position in response to closure of the push button switch I6, a'circuit was established for the actuating coil of the lookout relay 82 which circuit is traced from the lower side of the supply source II through the contacts of the master switch I3, conductors 34, 35 and 4|, upper concircuit independent of the upper contacts of control relay I5 so that when the upper contacts of control relay I5 were subsequently opened in response to deenergization of the relay 2 I, the lockout relay 32 remained closed. In its closed position, lockout relay 32 maintains its lower contacts open and thereby prevents reenergization of the forward contactor I2 until the lockout relay 32 is deenergized by actuating the master switch I3 to its open position in which it is illustrated.

In the modification of Fig. 2, the single core type relay 2I is replaced by a balanced voltage type relay 43 illustrated as comprising a pivoted arm member 3a to which two core members 43s and 43 in turn are pivoted as illustrated. The coils 43d and 3e are similar respectively to the coils 2Ib and 2I of Fig. 1 and are similarly connected through rectiflers 44 and 45 to the secondary and primary terminals of the motor. The contacts 43: correspond to the contacts 2Id and 2I1 of the relay 2I of Fig. 1. The coils 43d and 3e and resistors in the balanced voltage relay are so proportioned that at the instant of plugging, the contacts 43: will close, and not until the voltage generated across the slip rings of the motor has decreased to one-half of the locked rotor voltage, will the contacts of the relay open.

The operation of the modification of Fig. 2 is identical with the operation of the system of Fig. 1 described in the foregoing.

Although in accordance with the provisions of thepatent statutes the principle of this invention has been explained and the structure described and explained which embodies the best mode in which it is contemplated applying the principle of the invention, it will be understood that the apparatus and elements and connections shown and desscribed are merely illustrative and that the invention is not limited thereto, since alterations and modifications; will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention o from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:-

1. A control system 'for alternating current motors having primary and secondary windings comprising means for establishing reverse power connections to brake said motor, and means controlled in response to the voltage supplied to the primary winding and the voltage 01' said. secondary winding for interrupting said reverse power connections at substantially zero speed of said motor.

2. A control system for an alternating current motor having a primary winding and a secondary winding comprising means for establishing 'reverse power braking connections to the primary winding of said motor, and means responsive to the voltage of said secondary winding and the voltage supplied to said primary winding for interrupting said reverse power connections comprising an electromagnetic switching device having electrical connections to both said windings.

3. A control system for an alternating current motor having a primary winding and a secondary winding, comprising means for establishing reverse power braking connections from a source of power to said primary winding, and means responsive to the voltage supplied to said primary winding and the voltageof said secondary winding for interrupting said connections at substantially zero speed of said motor comprising an electromagnetic switching device having electrical connections to both said windings and rectitying means included in said electrical connections.

4. A control system for alternating current motors having a primary winding and a secondary winding comprising means for connecting said motor to a supply source, means for establishing reverse power braking connections for said motor, and means responsive to the voltage of said secondary winding and the voltage supplied to said primary winding for interrupting said reverse power connections at substantially zero speed of said motorcomprising an electromagnetic switching device having a core member and a pair of coils arranged in opposition thereon, electrical connections from one of said coils to the primary winding of said motor and from the other of said coils to said secondary winding and rectifying means included in said connections for eliminating the effect of the frequency of said voltages.

5. A control system for an alternating current motor having a primary winding and a secondary winding comprising a contactor for connecting the primary winding to a supply source for rotation in a forward direction, a second contactor for connecting said primary winding to said source for rotation in the reverse connection, means for actuating said first contactor to disconnect said primary winding from said source and for actuating said second contactor to connect said primary winding to said source to establish reverse power braking connections for said motor, and means for actuating said second contactor to interrupt said braking connections at substantially zero speed of said motor comprising an electromagnetic switching device having a core member and a pair of coils, each connected to one of said windings and arranged on said core so that their magnetic fluxes are in opposition.

6. A control system for alternating current motors having a primary winding and a secondary winding, reversing switching mechanism for said motor comprising forward and reverse electromagnetic contactors, means for actuating said forward contactor to connect said primary winding to a source, a switching device for deenergizing said forward contactor thereby to disconnect said primary winding from said source and for establishing an energizing circuit for said reverse contactor thereby to actuate said reverse contactor to establish reverse power braking connections for said motor, and means for deenergizing said reverse contactor to interrupt said reverse power braking connections at substantially zero speed of said motor comprising an electroresponsive device having a first coil connected to be responsive to the voltage supplied to the primary winding of said motor, an opposing coil connected to be responsive to the voltage of said secondary winding and contacts controlled by said coils for maintainingsaid energizing circuit for said reverse contactor upon establishment of said reverse power connections and for interrupting said energizing circuit when the magnetization of said opposing coil becomes equal to the magnetization of said first coil.

'7. A control system for an alternating current motor having a primary winding and a secondary winding comprising in combination a forward contactor and a reverse contactor, means for ac-r contactor to establish reverse power braking connections for said motor, and means for interrupting said reverse power braking connections at substantially zero speed of said motor comprising a balanced type relay having a pivoted member provided with a pair of cores connected to said member on opposite sides of the pivot and a coil surrounding each of said cores, and electrical connections from one of said coils to said primary winding and from the other of said coils to said secondary winding.

8. A control system for an alternating current motor, having a primary winding and a secondary winding, comprising in combination, reversing switching mechanism for said motor comprising forward and reverse electromagnetic contactors, means for actuating said forward contactor to connect said primary winding to a source, means for actuating said forward contactor to disconnect said primary from said source and for actuating said reverse contactor to connect said primary winding to said source to establish reverse power braking connections for said motor, and means for actuating said reverse contactor to interrupt said connections at substantially zero speed of said motor comprising an electromagnetic switching device for controlling the energization of said reverse contactor having electrical connections to said windings, interlock contacts controlled by said reversing switching mechanism for establishing said electrical connections only when said braking connections are established,- and rectifying means in said electrical connections.

9. A control systemfor an alternating current motor having a primary winding and a secondary winding comprising forward and reverse contactors, means for actuating said forward contactor to connect said primary winding to a source for rotation in the forward direction, means for actuating said forward contactor to disconnect said motor from said source and for actuating said reverse contactor to establish reverse power braking connections from said primary winding to said source, and a balanced voltage type relay for effecting the actuation of said reverse contactor at substantially zero speed of said motor, said relay having a pivoted arm member, a pair of cores attached thereto on opposite sides of said pivot, a coil surounding each of said cores, contacts actuated by the movement of said pivoted member/and electrical connections from one of said coils to said primary winding and from the'other of said coils to said secondary winding, said coils being so proportioned that their magnetomotive forces are substantially equal at zero speed of said motor whereby said contacts are actuated to deenergize said reverse contactor.

10. A control system for alternating current motors having a primary winding and a secondary winding comprising reversing switching mechanism for said motor, means for actuating said switching mechanism to connect said primary winding to a source for rotation in the forward direction, means for actuating said mechanism to disconnect said motor from said source and for establishing reverse power braking connections for said motor, and means for controlilng said switching mechanism to interrupt said reverse power braking connections at substantially zero speed of said motor comprising an electroresponsive device having electrical connections to said secondary winding and rectifying means included in said connections for eliminating frequency effects from the voltage supplied to said electroresponsive device.

11. A control system for an alternating current motor having a primary winding and a secondary winding comprising a forward contactor and a reverse contactor, means for actuating said forward contactor to connect said primary winding to a source for rotation in the forward direction, a stop control switch for actuating said forward contactor to disconnect said primary winding from said source and for actuating said reverse contactor to establish reverse power braking connections for said motor, and means for actuating said reverse contactor to interrupt said connections comprising an electroresponsive relay having an actuating coil and a pair of normally open contacts in parallel with said stop control switch, auxiliary interlock contacts actuated by said reversing contactors for connecting said coil to said secondary winding so that said device closes said normally open contacts when said braking connections are established and opens said contacts at a predetermined motor speed, and rectifying means included in the connections between said coil and said secondary winding for eliminating frequency effects from the voltage supplied to said coll.

ALLAN J. WADE. 

